The present invention relates to an aqueous ink composition, more particularly an aqueous ink composition which has excellent water resistance on paper and aging resistance as inks for writing tools, plotter pens or ink jet printers.
A variety of investigations and developments have been hitherto carried out for the purpose of affording water resistance on paper to written matter. Pigment type aqueous inks containing pigments are popular, but these inks lack stability when in the form of a dispersion and exhibit problems of aging resistance manifested by sedimentation of the pigment particles.
The present inventors have conducted research on the so-called aqueous ink compositions using a water soluble dye which has excellent aging resistance as compared with pigments and have excellent water-resistance on paper.
The present inventors have discovered that an anionic dye ink to which has been added a sparingly water-soluble compound containing at least one oleophilic group selected from an aromatic ring or an aliphatic hydrocarbon having 6-12 carbon atoms and at least one hydrophilic group selected from a group having at least one atom such as O, N or S remarkably improves the water resistance on paper.
However, the aforementioned compound is sparingly soluble in water, so that only a dispersion type ink can be obtained. On the other hand, the present inventors have found that among the aforementioned compounds, the compounds having a solubility of 5% by weight or more in ethanol at 20.degree. C. are preferred as they are solubilized very easily with an anionic dye having a surface tension in the range of 60 dyn/cm or less at 20.degree. C. measured as a 0.1% by weight aqueous solution.
Applicants have discovered an aqueous ink which has excellent aging resistance and has an excellent water-resistance on paper.